SKI and SnoN proteins have been shown to inhibit TGF-beta signaling, acting both as transcriptional co-repressors in the cell nucleus, and as sequestrators of SMAD proteins in the cytoplasm. TGF-beta, on the other hand, induces rapid, proteasome-mediated, degradation of both proteins. How elevated SKI and SnoN protein levels co-exist with active autocrine TGF-beta signaling in cancer cells is yet to be understood.

In this study, we found elevated SKI and SnoN protein levels in a panel of melanoma cell lines, as compared to normal melanocytes. There was no correlation between SKI protein content and the capacity of melanoma cells to invade Matrigel, to form subcutaneous tumors, or to metastasize to bone after intracardiac inoculation into nude mice. Nor did we find a correlation between SKI expression and histopathological staging of human melanoma. TGF-beta induced a rapid and dose-dependent degradation of SKI protein, associated with SMAD3/4 specific transcriptional response and induction of pro-metastatic target genes, partially prevented by pharmacologic blockade of proteasome activity. SKI knockdown in 1205Lu melanoma cells did not alter their invasive capacity or transcriptional responses to TGF-beta, and did not allow p21 expression in response to TGF-beta or reveal any growth inhibitory activity of TGF-beta.

Despite high expression in melanoma cells, the role of SKI in melanoma remains elusive: SKI does not efficiently interfere with the pro-oncogenic activities of TGF-beta, unless stabilized by proteasome blockade. Its highly labile nature makes it an unlikely target for therapeutic intervention.

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